Heat-resisting chromium-molybdenum-vanadium steel

ABSTRACT

A heat-resisting alloy steel which is basically a 1 percent chromium-molybdenum-vanadium steel, with the addition of at least one element selected from the group comprising titanium, tantalum and niobium in the range from 0.03 to 0.15 total percentage by weight, from 0.002 to 0.010 percent by weight boron and from 0.5 to 3.0 percent by weight cobalt. High-creep strength, rupture ductility and tensile strength properties are developed by austenitizing the steel in the range 950* C. to 1,060* C., hardening by cooling, and tempering in the range 600* C. to 700* C. for from 3 to 60 hours.

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i 1 we Stts ate Inventors Kenneth Arnold lRitlal;

John McCann, both of Yorkshire, England Appl. No. 738,103 Filed June 19,1968 Patented Oct. 26, 1971 Assignee English Steel Corporation LimitedYorkshire, England Priority June 29, 1967 Great BritainlllllEAT-RESlSTING CHROMIUM-MOLYB1DENUM- VANADIUM STEEL 6 Claims, NoDrawings U.S. Cl 75/128 B, 75/128 F, 75/128 V, 75/128 W lint. Cl C22c39/20 Field of Search 75/l28.4,

[56] Relierences Cited UNITED STATES PATENTS 2,880,085 3/1959 Kirkby75/128.6 2,968,549 1/1961 Brady 75/128.6 3,008,820 11/1961 Hurley 75/128.6

Primary ExaminerHyland Bizot Attorney-Stevens, Davis, Miller& MosherABSTRACT: A heat-resisting alloy steel which is basically a 1 percentchromium-molybdenurn-vanadium steel, with the addition of at least oneelement selected from the group comprising titanium, tantalum andniobium in the range from 0.03 to 0.15 total percentage by weight, from0.002 to 0.010 percent by weight boron and from 0.5 to 3.0 percent byweight cobalt. High-creep strength, rupture ductility and tensilestrength properties are developed by austenitizing the steel in therange 950 C. to l,060 C., hardening by cooling, and tempering in therange 600C. to 700C. for from 3 to 60 hours.

BACKGROUND OF THE INVENTION Conventional 1% chromium-molybdenum-vanadiumsteels when used for aeroengine turbine shafts although havingreasonable tensile strength of the order of 40 to 50 tons per squareinch, exhibit poor high-temperature creep resistance leading to failureof the shaft from creep. Attempts to increase the creep resistance ofaeroengine turbine shafts by manufacturing them from 3%chromium-molybdenum-vanadium steels, although increasing the tensilestrength of the shafts to up to 85 tons per square inch, have failed toincrease the creep resistance to a satisfactory degree. Moreover theaforesaid conventional low-alloy steels have exhibited inferior creepproperties when used for steam and gas turbine components subject to acombination of high temperature and stress, and when used for otherrotating components such as discs, and components subject to stress,such as bolts, fasteners and tubes. 1

Furthermore, the aforesaid conventional low-alloy steels have restrictedranges of tensile properties, may not retain their tensile propertiesover a wide tempering range and are not usually suitable for themanufacture of large sections due to variation of tensile propertiesthroughout the section coupled with low through hardenability.

It is accordingly an object of this invention to provide a new andimproved heat-resisting alloy steel having an optimum combination ofhigh-temperature creep strength, rupture ductility and tensile strength,superior to that previously attainable with low-alloy heat-resistin gsteels.

It is another object of this invention to provide a new and improvedheat-resisting alloy steel which can be heattreated to a wide range oftensile properties.

It is a further object of this invention to provide a new and improvedheat-resisting alloy steel which is exceptionally resistant totempering, and thereby maintains good tensile properties over a widerange of tempering.

It is still another object of this invention to provide a new andimproved heat-resisting alloy steel which attains good tensileproperties equally well in small or large sections and has high throughhardenability.

SUMMARY OF THE DESCRIPTION The foregoing objects are accomplished byproviding a heat-resisting alloy steel containing by weight from 0.15%to 3.5% carbon, not more than 0.35% silicon, from 0.4% to 1.0%manganese, from 0.4% to 1.0% nickel, from 0.7% to 1.4% chromium, from0.5% to 1.5% molybdenum, and from 0.20% to 0.60% vanadium, and furthercontaining at least one element selected from the group comprisingtitanium, tantalum and niobium in the range from 0.03% to 0.15% total byweight, from 0.002% to 0.010% by weight boron, and from 0.5% to 3.0% byweight cobalt to improve the creep strength, rapture ductility andtensile strength of the steel, the balance, except for impurities andincidental constituents which include from to 0.040% by weight sulfurand from 0% to 0.040% by weight phosphorus, being iron. The steel of theinvention is heat treatable by austenitizing in a temperature range 950C. to 1060 C., hardening by cooling, and tempering in a temperaturerange 600 C. to 700 C. for from 3% to 60 hours.

DESCRIPTION OF PREFERRED EMBODIMENTS Other objects, features andadvantages of the invention will become apparent on reading thefollowing detailed description.

By way of example, steels produced in accordance with the invention andhaving constituents in a preferred quantity range and in specificquantities within the range are set out in the following as Type A andType B respectively:

Constituent Type A of Type B Z ol'totul total weight weight (vacuum at:

remelted) Carbon from 0.22 to 0.28 0.23 Silicon 0.30 mnx. 0.30 Manganesefrom 0.5 to 0.7 0.39 Nickel from 0.5 to 0.7 0.68 Chromium from 0.9 to1.1 1.07 Molybdenum from 0.65 to 0.85 0.85 Vanadium from 0.40 to 0.500.50 Cobalt from 1.5 to 2.5 2.18 Boron from 0.004 to 0.008 0.005Titanium,

Niobium and/or from 0.08 to 0.10 0.06 Titanium Tantalum In each of theforegoing types of steel the balance of the constituents is iron exceptfor impurities and incidental constituents, such as sulfur which ispresent in an amount not exceeding 0.015% by weight in Type A and of0.009% by weight in Type B, and phosphorus which is also present in anamount not exceeding 0.015% by weight in Type A and of 0.009% by weightin Type B.

The addition of the elements cobalt, titanium, niobium or tantalum, andboron in combination to what is basically a 1% Cr-Mo-V type of alloysteel, jointly stabilizes the structure, in particular thevanadium-carbide-hardening phase, and renders the alloy steel suitablefor high-temperature applications. Moreover, in steels of the presentinvention, it is preferable to keep the carbon content in the range0.22% to 0.28% by weight to ensure optimum creep strength and creepductility properties. Also the manganese and nickel contents shouldpreferably to controlled within the stated ranges so that the combinedmanganese and nickel content in the steel is not greater than 1.5% byweight, to prevent the steel reverting to an austenitic structure ontempering. Furthermore, the chromiumand molybdenum-hardening elementsare kept within the stated ranges to prevent chromium and molybdenumcarbides detrimentally replacing vanadium carbide, which is acreep-strength-improving phase produced by the vanadium in the steel.

It is to be noted that the steel contains at least one of the group ofstrong carbide-forming elements comprising titanium, tantalum andniobium within the range 0.03% to 0.15% by weight, and in fact it ispossible to have these three alloying constituents either individuallyor in combination within the stated range. Indeed the combined use ofcobalt, a strong carbide former (titanium, tantalum and niobium) andboron considerably improves the creep properties of the steel over theconventional 1% chromium-molybdenum-vanadium and 3%chromium-molybdenum-vanadium steels in common usage. In fact, thecombination of creep strength, rupture ductility and tensile strengthattainable with an alloy steel of the invention within the stated rangesis superior to that previously developed in low-alloy heawesistantsteels.

Steel produced with the invention is heat-treated by austenitizing inthe temperature range 950 C.l060 C., and is then hardened by eithercooling in air, steam, water mist, or oil. The desired mechanicalproperties are then attained by tempering the steel in the range 600-700C. for 3 to 6 hours. In this way the steel can be heattreated to a widerange of tensile properties, from 50 to tons/square inch ultimatetensile strength. Furthermore, a steel produced in accordance with theinvention is exceptionally resistant to tempering, in that the tensileproperties are maintained over a wide range of tempering, has goodhot-strength properties up to 600 C., and is capable of being surfacehardened by nitriding to give a good case.

The properties of the steel can be attained equally well in small andlarge sections, due to the high through hardenability of the steel.lnparticular the tensile properties may be attained with a range ofmartensitic and/or bainitic structures. The optimum structures forresistance to creep strain are upper bainitic structures. Whenheattreated to an 85 tons per square inch condition, the alloy steel ofthe invention is capable of withstanding a stress of 20 tons per squareinch for hours at 550 C. while exhibiting a total plastic strain of lessthan 0.1%.

For test purposes, l-inch diameter bars were prepared from steel withinthe aforesaid ranges, hardened for 1 hour at 1050 C. and air cooled. Theeffect of tempering treatment on mechanical properties is shown by thefollowing test results, which illustrate the wide range of usefulproperties and great intrinsic resistance to softening of a steelproduced according to the invention.

EFFECT OF TEMPERING TREATMENT ON MECHANICAL PROPERTIES Proof stressUltimate (tons/ tensile sq. in.) strength Percentage Tempering (tons/Percentage reduction treatment 10% 20% sq. in.) elongation of area 4hours 600 C- 75.0 70.1 87. 3 19. 02. 5 8 hours 600 C 74. 6 80.8 86.0 17.5 59. 0 20 hours 600 C 73.4 78.0 85. 7 15.0 51.0 4 hours 650 C 70.1 72.076. 9 19.0 50. 6 8 hours 650O 67.2 68.4 73.5 10.0 50.2 20 hours 650 C62. 2 63.7 70.4 17.5 47. 0 4 hours 700C 61.4 63.1 70.0 19.0 57.5 8 hours700 C 58. 3 50. 2 65. 9 21.0 61. 7 20 hours 700 C 51. 6 52. 3 00. 3 20.0 60. 2 60 hours 700 C 48.6 40. 3 57.3 19. 0 (10.2

As aforesaid, steels produced in accordance with the invention have goodhot-strength properties up to 600 C. For test purposes, samples of thesteel were austenitized at 1050 C. for 1 hour, air cooled, tempered at625C. for 8 hours, and air cooled. The effect of test temperature uponmechanical properties is shown by the following rest results:

1n steels produced in accordance with the invention, lowtemperaturetransformation products such as martensite are preferred, if the bestimpact strength and creep ductility is required. This may bedemonstrated by the following test results obtained from l-inch diameterbars of steel of the invention, which were oil-hardened from 975 C. andtempered for 8 hours at 625C.

MECHANICAL PROPERTIES AT ROOM TEMPERATURE P r c Ult' t t '1 i i roo sress ima e ensi e e onga ion (tons/sq. in.) strength at g gg ia 0.2%(tons/sq. 111.) 4 Area area CHARPY VNOTOH IMPACT TEST N otch-tensilestrength (tons/sq. in.) after 300 hrs. at-

Notch-tensile Impact value strength (foot-pounds) (tons/sq. in.) 400 C.500 0. 550 C.

CREEP PROPERTIES AT 550 C.AN1) 20 TONS/SQ. IN.

These properties exhibited by the steel produced according to theinvention are superior to those previously attained on low-alloy steels,and in some respects may be compared with those commonly obtained on thehigh-chromium steels (12% chromium, molybdenum, vanadium, niobium).However, the steel of the present invention is more resistant totempering, is capable of being nitrided, and is more economic than thehighalloy steels.

The above steels should be particularly useful for the manufacture ofaeroengine turbine shafts, but are also suitable for a variety ofapplications. In particular the aforesaid ste'els are suitable for usein steam and gas turbines, where the component fabricated therefrom issubject to a combination of high temperature and stress. Particularexamples are rotating components, shafts and discs, and componentssubject to stress such as bolts and fasteners, and tubes.

It is to be understood throughout this specification that weights givenin tons, refer to long tons.

While preferred embodiments have been described, it is to be understoodthat various modifications and changes may be made without departingfrom the spirit and scope of the invention. What is claimed is: v

1. Heat-resisting alloy steel consisting essentially of by weight from0.15% to 0.35% carbon, not more than 0.35% silicon, from 0.4% to 1.0%manganese, from 0.4% to 1.0% nickel, from 0.7% to 1.4% chromium, from0.5% to 1.5% molybdenum, and from 0.20% to 0.60% vanadium, and furthercontaining at least one element selected from the group comprisingtitanium, tantalum and niobium in the range from 0.03% to 0.15% total byweight, from 0.002% to 0.010% by weight boron, and from 0.5% to 3.0% byweight cobalt to improve the creep strength, rupture ductility andtensile strength of the steel, the balance, except for impurities andincidental constituents which include from 0% to 0.040% by weight,sulfur and from 0 to 0.040% by weight phosphorus, being iron.

2. Heat-resisting alloy steel according to claim 1 containing at leastone element selected from the group comprising titanium, tantalum andniobium in the range from 0.08% to 0.10% by weight.

3. Heat-resisting alloy steel according to claim 1 containing by weightfrom 0.004% to 0.008% boron.

4. Heat-resisting alloy steel according to claim 1 containing by weightfrom 1.5% to 2.5% cobalt.

5. Heat-resisting alloy steel according to claim 1 in which the combinedmanganese and nickel content is not greater than 1.5% by weight.

6. Heat-resisting alloy steel according to claim 1, containing byweight, from 0.22% to 0.28% carbon.

2. Heat-resisting alloy steel according to claim 1 containing at leastone element selected from the group comprising titanium, tantalum andniobium in the range from 0.08% to 0.10% by weight.
 3. Heat-resistingalloy steel according to claim 1 containing by weight from 0.004% to0.008% boron.
 4. Heat-resisting alloy steel according to claim 1containing by weight from 1.5% to 2.5% cobalt.
 5. Heat-resisting alloysteel according to claim 1 in which the combined manganese and nickelcontent is not greater than 1.5% by weight.
 6. Heat-resisting alloysteel according to claim 1, containing by weight, from 0.22% to 0.28%carbon.